In order to certify equipment for installing on board aircraft, it is necessary in particular to verify the behavior of that equipment when it is placed under environmental conditions similar to those that it might encounter on aircraft. The equipment is subjected to a battery of qualification tests, during which it is verified whether the equipment, when subjected to such environmental conditions, functions normally, or functions in an acceptable degraded mode, or is not damaged, or indeed disturbs the operation of other pieces of equipment. These tests generally include climatic tests (temperature, humidity, etc.), mechanical tests (vibration, impacts, etc.), electromagnetic compatibility tests, tests of ability to withstand lightning striking the aircraft, etc.
Among the electromagnetic compatibility tests, there are tests for susceptibility of radiofrequencies seeking to verify the behavior of an electrical member when it is exposed to a certain level of radiofrequency energy. These tests also include conductive susceptibility tests during which currents are injected into the cables connected to the electrical member at frequencies that are swept over a broad spectrum.
In order to verify the ability of the electrical member to withstand lightning strikes, tests are provided that are similar to the conductive susceptibility tests, and also current injection tests that are performed directly via the connectors of the equipment.
Certain electrical members that perform functions that are particularly critical, e.g. an electrical member used in an electrical flight control system, are required not only to be capable of withstanding such injected currents, but also to be capable of operating normally while such injection is taking place.
Thus, there are requirements for an electrical member to be capable of sending and receiving data in normal manner over a digital differential communications line even when a current of several hundreds of milliamps is injected into the line at frequencies varying over the range 100 kilohertz (kHz) to 400 megahertz (MHz). The injected current leads in particular to high levels of common mode disturbance that propagate along the conductors of the line.
In order to enable the electrical member to pass such a test successfully, it might be envisaged mounting a connection device on the electrical member for connecting it to the communications line via a first transformer having two windings connected in series with the respective conductors of the line, and a second transformer of the center-tapped type for electrically isolating the electrical member from the communications line. The second transformer would have a winding with two conventional connection terminals together with an additional connection terminal to the midpoint of the winding. Thus, a common mode disturbance generated by current being injected would be filtered by the transformers and discharged to frame ground of the electrical member via the center tap.
The use of such transformers would be effective, except when the common mode disturbance propagates at a frequency corresponding to a resonant frequency of the connection device due to an inductive component or the first transformer and a capacitive component of the second transformer. At such a resonant frequency, which gives rise to a great reduction in the impedance of the reception member and thus to a large increase in common mode current, there is a severe risk of a common mode disturbance damaging the components of the electrical member, or indeed interrupting communication.